Fire and Water

Surprising new research shows that around 20% of water, otherwise available to the Murray River alone, can be withheld for decades. Bushfires in the Snowy Mountains periodically strip the highlands bare of vegetation – the regrowth that follows soaks up more water than a mature forest, withholding large amounts of water for as long as 40 years.

Paul Willis visits the high country to see what might be done to avoid this phenomena re–occurring.

Transcript

Bushfires one minute. Ice and snow the next. And occasional showers in between.

Yet it's this rain and snow that the Murray depends on.

Now we've all heard about a number of problems with the quality and quantity of water in the whole Murray Darling system. But the problem I'm about to explore starts before a single drop of water even gets into the river. And the source of that problem is completely unexpected.

Believe it or not, bushfires are probably the most underestimated factor in the future of water supply, particularly up here in the High Country.

It seems that more bushfires equals less water downstream. Much less.

Professor Mark Adams: We're not talking about one gigalitre, we're talking tens of gigalitres or even hundreds of gigalitres when you look at the areas that are now being burnt.

Narration: Five years ago huge areas of The High Country were burnt to a crisp in some of the most devastating bushfires we've ever seen here.

What this means for water supply goes something like this....

In a mature forest, when it rains, the trees, shrubs, and grasses take what they need. The rest of the water trickles off to make the streams and rivers or enters the ground water.

But when a bushfire comes along, the vegetation is suddenly and completely eradicated. For a year or two, water streams off at great rate.

But as the bush regrows, the burgeoning new vegetation literally sucks the water out of the ground.

The result: less run off for the rivers. And this effect can last for decades.

Prof Mark Adams at the University of NSW wants to put some serious data into the theory. So he and his colleagues are measuring every conceivable parameter that relates the growth of vegetation to fire and water.

But how do you measure the way plants grow? You put a glass box around them and record the gases coming out.

Dr Paul Willis: So what exactly do these boxes sample?

Professor Adams: Yes, we take the sample of the gas and then we, it runs through these lines, all the way back to the laboratory and we measure the CO2, methane, and N2O.

Paul: And how often does it ... ooh! What's going on now?

Professor Adams: Well, it's going to open because when we're not sampling, we want to keep the atmosphere the same as it is outside. So when we sample they close, but most of the time they sit open

Narration: Dispersed throughout the nearby forests and grassy paddocks groups of students measure things like water stress and leaf transpiration as well as conducting infra-red gas analysis and laser measurements of evaporation.

Professor Adams: We're seeing the sort, the numbers that would indicate say a twenty per cent loss in stream flow from an average sub-catchment in the high country. Now that can equate to a lot of water.

Paul: It's quite a shock to see just how much of the high country was burnt in that terrible summer of 2003.

And if you think it looks bad, it's about to get worse

Back in Canberra, they're looking closely at bush fires. Geoff Cary at the ANU is making some bushfire predictions.

Dr Geoff Cary: We've built a, a landscape computer model and this model here is for the ACT region.

The warmer colours indicate shorter intervals between fires, so perhaps ten to thirty years between fire events. The cooler colours or the green colours indicate much longer intervals between fires, perhaps as long as eighty, ninety or a hundred years between fires.

Paul: That's for the current climate. But what if Geoff factors in the sort of temperature increase we might expect for global warming?

Dr Cary: We've re-run the model and we can look at the number of fires in the landscape that will occur under that changed climate. And the effect is quite dramatic.

Paul: Wow!

Dr Cary: We're seeing a significant proportion of the landscape that's burning more frequently than every ten years, indicated by this red colour. And if we link that with the rapid regeneration phase immediately after fire that'll certainly have implications for water use.

Narration: It looks like one the biggest drains on water supplies to the Murray Darling is only going to get bigger. But, unlike many other factors, there is something we can do about the burning High Country and its effects on water supply - and that's with the highly controversial and emotionally charged practice of frequent, controlled burns.

Professor Adams: So if we can use fire to control fuels, without creating mass regeneration, then we will suffer less, if you like. We won't lose as much water. Whereas if we let major fires run unchecked, where we have very high intensity fires over millions of hectares, then we create mass regeneration and that will really move things to our, the upper limit of our estimates, of the loss of water.